1. Field of the Invention
The invention relates to a method for coating a porous electrode for electrochemical processes with an activation layer which covers the electrode surface at least in part and contains metals or compounds of metals of the platinum group.
2. Description of the Prior Art
For activating electrodes for electrochemical processes, for instance, of anodes for the chlorine-alkali electrolysis, of a material which is resistant to the products of the electrolysis and forms a passivating layer under the conditions of the electrolysis, numerous methods have become known, the purpose of which is essentially to anchor a platinum metal or compounds containing platinum metals mechanically firmly on the electrode or the electrode core in an electrochemically effective degree of dispersion. It is known, for instance, from German Published Prosecuted Application No. 11 55 762 to coat degreased and pickled titanium sheets by electroplating with a platinum metal and to heat the sheets in a first thermal cycle in an inert atmosphere and in a second cycle in an oxidizing atmosphere to a temperature of 800.degree. C. With this treatment, a firmly adhering activation layer and at the same time improved protection of the electrode material is obtained by reaction of the titanium core in rutile which core is exposed in the pores of the activation layer which is a thin barrier layer of titanium oxide. This and other coating methods that have become known are not as suitable, however, for porous electrodes, for instance, for sintered electrodes according to German Published Non-Prosecuted Application No. 23 05 175 or electrodes of titanium suboxide according to German Published Prosecuted Application No. 24 05 010. The adhesion of the activation layers is particularly favorable with porous electrodes and for many electrochemical processes, the large surface of the electrode is an advantage. However, in coating the electrode with an activation layer, losses of activating agent occur if the known methods are used, since the platinum metals or platinum metal compounds are in part also deposited in the pores of the electrode which are away from the surface, and the surfaces of which do not participate in the electrochemical reactions. The loss is particularly great if the activating agent is precipitated from solutions and is not so great if the activation layer is electrodeposited. Electrodeposited layers, on the other hand, are less suitable because of their dense structure. It has also been proposed (for instance, German Published Prosecuted Application No. 16 71 422) to apply the metals or metal compounds to the electrode surface from finely dispersed suspensions. Because of the great difficulty in obtaining uniform distribution and good adhesion of the applied activating substances, the above-described coating methods are preferred on a technical scale.